There is a need for longer lived tools useable in oxidizing environments. Such tools are frequently required to maintain a high degree of polish during long production runs so that the workpiece, after removal from the tooling, has a smooth, blemish free surface. The glass industry, which is characterized by tight finish specifications, is a prime example of such an application, and the invention will be described in connection with this industry, though it will be understood that the invention has application to certain tooling applications in the plastic molding, non-ferrous die casting, and other corrosive environment and elevated temperature applications.
Specifically, in the glass industry, and particularly that portion of it devoted to the production of television tubes, there is a demand for mold materials which are resistant to oxidation when in contact with molten glass for long periods of time. This stems from the fact that after a glass television viewing screen has been formed, usually in a three part mold, at least one of the mold parts must be retracted to a part clearing position while in contact, at least briefly, with the formed screen; in other words, sliding contact occurs between the just formed workpiece, and at least one component of the multi-component mold. It will be understood that all components of the mold have a smooth, highly polished surface so that the formed screen will have no surface imperfections or irregularities which would result in a distortion of light transmitted through the screen. If any component of the multi-part mold, and particularly the retractable component which makes sliding contact with the just formed workpiece, has a rough surface, the surface of the just formed, but still deformable, part will reflect the surface discontinuity of the mold, and the formed part will be unacceptable. The oxidation which forms on the tool is sufficient, after a production run of a duration shorter than desired, to create a rough surface on the formed workpiece and subsequent rejection of the workpiece.
At the present time, the materials of choice for mold components in the glass industry are martensitic stainless steels. The 420 type has been the preferred choice for molds used in glass applications due to its strength and wear resistance properties in addition to oxidation and corrosion resistance.
Unfortunately, the oxidation and corrosion resistance of 420 type stainless steel is not sufficiently capable of withstanding the temperatures and oxidizing environment in the glass industry for extensive times. Because of these limitations, glass mold components manufactured from 420 type stainless steels must be periodically removed from service to remove the oxide build-up that forms on them over time.